Dynamic tomography is a technique for producing a set of radiographic records of an object which, when properly combined, enable the reproduction of images of any selected plane or lamina within the volume of interest in a manner which obliterates interfering shadows and thus produces a virtual focus of the image of the selected plane. Dynamic tomography, which is sometimes referred to as varible depth laminagraphy, is described in U.S. Pat. No. 3,499,146 granted to Albert G. Richards on Mar. 3, 1971 for "Variable Depth Laminagraphy with Means for Highlighting the Detail of Selected Lamina."
As described in this reference, dynamic tomography is carried out by a tomographic movement of an X-ray source and film, with the effective pivot point of the linkage between the source and film being disposed in a plane through the object under examination. Multiple exposures are made at different angles through the object, with separate images being recorded on separate films for each different angle, to produce a set of radiographs. The set of radiographs may then be superimposed in a stack for viewing and, with the radiographs aligned so that the images of a given point in the object all coincide, all other points or object details in the same plane, will be revealed in an unobstructed view; images of all points lying in other planes, even closely adjacent planes, will be obliterated. Assuming, for instance, the use of cut films with the image of a given point in the reference plane falling at the center of each film for each exposure angle, a virtual focus of the reference plane is achieved and the images of the given point are superimposed when the radiographs are superimposed directly on top of each other. If the given point lies between the reference plane and the film, the images of the given point on the respective radiographs will be superimposed and the plane through the given point will be brought into virtual focus when the radiographs are shifted relative to each other in one direction parallel to the tomographic movement, i.e. the track direction. On the other hand, if the given point lies between the reference plane and the source, the images of the given point will be superimposed with the respective radiographs shifted relative to each other in the opposite direction parallel to the tomographic movement. Any desired plane may be brought into virtual focus by relative shifting of the set of radiographs to superimpose the images of a point lying in the plane of interest. In this relationship of the radiographs, the multiple images on the different radiographs of the same point reinforce each other, while a given point in any other plane is imaged on different radiographs at nonaligned positions and the resulting image is, therefore, blurred to the extent that it merely forms a background for the points in the reference plane.
Although dynamic tomography is a proven and successful technique for demonstrating parallel planes through the depth of the body, it has heretofore been difficult to extend the principles of dynamic tomography to demonstrate other surfaces of different shape and orientation within the body. The earlier referenced Richards patent, U.S. Pat. No. 3,499,146, proposes a method for accomplishing the demonstration of arbitrary surfaces within a body. In particular, it requires that each of a plurality of laminar surfaces through the body be raster scanned. The scanned data is then digitized and electronically stored in accordance with a storage scheme that addresses each scan element in terms of the laminar image in which it appears and its position within that laminar image. In short, the scanning technique builds up a 3-dimensional Cartesian representation of the body. When an arbitrary surface is to be demonstrated, it is constructed by calling up the addresses of the scanning elements that represent the surface in the 3-dimensional representation of the body, and presenting them on a display. This method, while practicable, requires an extensive commitment in electronic scanning apparatus, storage means and display means for the demonstration of an arbitrary surface.
An objective of the present invention is, therefore, to extend the principles of dynamic tomography to provide a simplified manner of demonstrating and recording an arbitrary surface within a body.